1. Field of the Invention
The present invention relates generally to a method of fabricating an aperture plate for a roofshooter type printhead and in particular to a method of forming an aperture plate for a printhead, such as a thermal ink jet (TIJ) printhead, by orientation dependent etching.
2. Description of the Related Art
There are two general configurations for thermal drop-on-demand inkjet printheads. In one configuration, droplets are propelled from nozzles in a direction parallel to the flow of ink in ink channels and parallel to the surface of the bubble generating heating elements of the printhead, such as, for example, the printhead configuration disclosed in U.S. Pat. No. 4,601,777 to Hawkins et al. This configuration is sometimes referred to as "edge or side shooters". The other thermal ink jet configuration propels droplets from nozzles in a direction normal to the surface of the bubble generating heating elements, such as, for example, the printhead disclosed in U.S. Pat. No. 4,568,953 to Aoki et al. and U.S. Pat. No. 4,789,425 to Drake et al. This latter configuration is sometimes referred to as a "roofshooter".
In the "roofshooter" printhead disclosed in U.S. Pat. No. 4,789,425, the disclosure of which is herein incorporated by reference, the printhead comprises a silicon heater plate and a fluid directing structural member. The heater plate has a linear array of heating elements, associated addressing electrodes, and an elongated ink feed slot parallel with the heating element array. The structural member contains at least one recess cavity, a plurality of nozzles, and a plurality of parallel walls within the recess cavity which define individual ink channels for directing ink to the nozzles. The recess cavity and feed slot are in communication with each other and form the ink reservoir within the printhead. The ink the recess cavity. The feed slot is precisely formed and positioned within the heater plate by anisotropic etching. The structural member may be fabricated either from two layers of photoresist, a two stage flat nickel electroform, or a single photoresist layer and a single stage flat nickel electroform.
The roofshooter type of printhead has a number of advantages over the side-shooter geometry. First, the roofshooter type does not have any problem with refill, i.e., it can operate at much higher print rates than a sideshooter geometry printhead. In addition, roofshooters do not suffer from air ingestion problems. A sideshooter geometry printhead can ingest air through the nozzles into the ink channels which can cause printing errors. The roofshooter, on the other hand, can not ingest air through the nozzles into the ink channels due to the geometry of the printhead.
Another basic roofshooter type thermal ink jet printhead is disclosed in U.S. Pat. No. 4,791,440 to Eldridge et al. Eldridge shows a printhead wherein a heater array is located on a substrate. A nozzle plate with apertures is bonded on top of the substrate to form a printhead.
There are many methods and processes for fabricating a thermal ink jet printhead. In particular, there are a number of methods for fabricating a aperture plate for a roofshooter type printhead. For example:
U.S. Pat. No. 4,961,821 to Drake et al., the disclosure of which is incorporated herein by reference and assigned to the same assignee as the present application, i.e., Xerox Corporation, discloses a method of orientation dependent etching (ODE) for forming semiconductor wafers which are used for an aperture plate of a thermal ink jet printer.
U.S. Pat. No. 4,455,192 to Tamai discloses a method for the formation of a multi-nozzle ink jet printhead wherein areas on a single crystal silicon plate are doped with impurities to make those areas more etch resistant than a remainder of the plate. Then, a second silicon plate is grown on top of the first. Both plates are then etched at once by anisotropic etching to form an array of nozzles within the first plate and a trough within the second plate.
The Bassous article from the IBM Technical Disclosure Bulletin, Vol. 19, No. 6, November 1976, pp. 2249-2250, discloses a nozzle array in a mesa structure etched in single crystal silicon wherein the nozzle array is used for an ink jet printhead. A method of fabricating an array of nozzles comprises: 1) defining a pattern on the front side of the wafer and etching mesas in an anisotropic etching solution; 2) defining a nozzle array pattern; 3) performing a p+ diffusion to a required depth; 4) defining a pattern of windows on the back side of the wafer and anisotropically etching the wafer through to the p+ silicon; and 5) stripping the wafer. FIG. 1D shows an anisotropically etched wafer which has troughs on one side of the wafer and a number of apertures formed within the troughs.
The Galicki et al. article from the IBM Technical Disclosure Bulletin, Vol. 22, No. 7, December 1979, pp. 2860-2861, discloses a process for fabricating ink jet nozzles wherein a single crystal silicon substrate is anisotropically etched on one side to form a single opening. The other side of the substrate is then plasma etched to form an aperture as shown in STEP E.
U.S Pat. No. 4,169,008 to Kurth discloses a process for producing uniform nozzle orifices in silicon wafers wherein an aperture plate is formed by a two-stage anisotropic etching process comprising the steps of: 1) etching a front face of a silicon wafer anisotropically to form a pyramidal nozzle; and 2) etching the back face of the silicon wafer anisotropically to form an aperture which is aligned with the nozzle and forms a hole entirely through the wafer.
U.S Pat. No. 4,914,736 to Matsuda discloses a liquid jet recording head having multiple liquid chambers on a single substrate wherein an aperture plate, which may be fabricated from silicon, includes a trough. The trough has a plurality of apertures located within the trough. The trough is placed over the actuating circuitry.
A number of these methods use orientation dependent etching to produce an aperture plate for a thermal ink jet printhead, but these methods do not use a two stage orientation dependent etch.
Orientation dependent etching (ODE) is disclosed in U.S. Pat. No. 4,169,008 to Kurth, the disclosure of which is incorporated herein by reference. First, a silicon wafer is produced which has a major surface lying substantially in the "100" plane. Then, a suitable anisotropic etchant is used to etch a pattern in the silicon wafer. The anisotropic etchant works well normal to the "100" plane as opposed to lateral or parallel to the "100" plane. To control the etch, an etchant masking material is placed on both sides of the silicon wafer. Holes are then made in the masking material in a certain pattern and etching can commence. Thus, any pattern can be made in the masking material and the etched using ODE. The depth that the etchant goes through the silicon wafer depends on the amount of time the etchant continues. Also, the shape that the etchant etches out has a generally inward sloping shape which is caused by the orientation of the silicon in the wafer.